1. Field of the Invention
The present invention relates to a frequency overlay system. More particularly, the present invention relates to an apparatus and method for allocating frequency resources in a frequency overlay system.
2. Description of the Related Art
Under the Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard, each Mobile Station (MS) is allocated a frequency resource of 5˜20 MHz. Available frequency resources of a Base Station (BS) are divided into a plurality of Frequency Allocations (FAs) which are allocated to MSs serviced by the BS. In an IEEE 802.16m system, a one-dimensional frequency resource allocation technique is used due to its low complexity, low overhead, separate coding MAP message, and high link adaptation gain of each MS's MAP Information Element (IE).
Frequency Overlay (FO) has been devised to support MSs that use various bandwidths. To this end, a frequency resource of a BS is divided into a plurality of FAs, and an MS can access each FA according to availability of the bandwidth. Therefore, an FO system offers a separate coding, one-dimensional resource allocation algorithm that can efficiently operate on multiple FAs.
FIG. 1 illustrates a related FO system having two 10-MHz FAs. FIG. 2A schematically illustrates a related start-length allocation method for resource allocation, and FIG. 2B schematically illustrates a related tree-based allocation method for resource allocation.
As illustrated in FIG. 1, when a BS 140 in an FO system is allocated a frequency resource of 20 MHz, the frequency resource allocated to the BS 140 is divided into two 10-MHz frequency resources. Of the divided frequency resources, the first 10-MHz frequency resource is allocated to an MS_A 110 and the second 10-MHz frequency resource is allocated to an MS_C 130. Herein, the frequency resource allocated to the MS_A 110 is called FA#1, and the frequency resource allocated to the MS_C 130 is called FA#2. An MS_B 120 is allocated both the FA#1 and the FA#2. In this way, an MS can be allocated at least one FA.
The concept of a separate coding MAP message will be described below with reference to FIG. 1. IEs of the MAP message are encoded for each MS individually. Based on the MAP message, a BS adjusts Modulation and Coding Scheme (MCS) level of each MS. When separate coding is used, power for each IE is allocated according to the current channel state of a designated MS instead of an MS having the worst communication state. In the separate coding MAP message, each MAP IE is considered as an individual MAP message, and a Cyclic Redundancy Check (CRC) is added to the end of each MAP IE. Using individual MAP message can cause severe control overhead problem but this may be reduced by removing a Connection Identifier (CID) field for an MS from each MAP IE. In this case, in order to allow each MAP IE to be identified by a specific MS, each MAP IE is individually scrambled such that only an identified MS can decode the MAP IE. When the separate coding MAP message is used, a one-dimensional allocation technique, e.g. start-length and tree-based allocation methods, may be used, in which each MS is not required to detect MAP IEs of other MSs.
FIG. 2A illustrates a start-length allocation method. Frequency resources allocated to each MS are identified by a starting point 202 (206) and a length 204 (208) of a plurality of designated resource slots. FIG. 2B illustrates a tree-based allocation method. In the tree-based allocation method, all resource slots are expressed in a tree structure, and frequency resources allocated for each MS are defined as a tree node in the tree structure.
As illustrated in FIG. 2A, among all frequency resources allocated to a BS, slots 204 of #0 to #4 are allocated to an MS_A and slots 208 of #9 to #14 are allocated to an MS_B, in accordance with the start-length allocation method. In this case, a starting point 202 and a length (=5) 204 are notified to the MS_A, and a starting point 206 and a length (=6) 208 are notified to the MS_B.
As illustrated in FIG. 2B, the tree-based allocation method allocates BS frequency resources to nodes in the tree structure. For example, a node with a node ID=6, which is an upper node of nodes #1 and #2, are used for allocating slots #0 and #1. And the each number of 0, 1, . . . 4 indicates slot index.
In these start-length and tree-based allocation methods, there are two significant problems regarding MAP overhead of the separate coding MAP message. The first problem is a granularity problem. In other words, since each MAP IE is encoded as part of the MAP message, a plurality of 48-bit physical resource blocks are consumed for each MAP IE. In the second problem, because of characteristics of the separate coding, each MS cannot detect useful information from MAP IEs of other MSs.
Due to these two problems, excessive overhead is caused in the FO system. Furthermore, this overhead grows as the number of FAs used for resource allocations are increased. Therefore, there is a need for an improved frequency resource allocation method for a separate coding MAP message in order to reduce the overhead and to address the problem that the MAP overhead increases as the FO system uses additional FAs.